As an alternative source to gasoline refined from petroleum, methods for conversion of coal and/or methane to liquid phase fuels have been under consideration since the 1920's.
Such methods as have been devised for production of liquid phase fuels from non-liquid carbon containing materials, have to date been unable to compete economically with gasoline refined from petroleum. Such practice of synthetic gasoline production methods as has occurred to date (other than pilot plant studies) has been dictated by circumstances which override economic considerations.
In the 1930-40's, Germany produced liquid fuels from coal on a large scale. Such synthetic gasoline production was necessitated by Germany's lack of petroleum resources. Germany's synthetic production of gasoline from coal was accomplished by a processing technique known as the Fischer-Tropsch Synthesis (FTS). In FTS gasoline production, a carbon containing source material, typically coal or alternatively methane, is first converted to a gas stream containing carbon monoxide, carbon dioxide, hydrogen, water and impurities. The gas stream is then purified of water and adjusted in composition to a synthesis gas which is thereafter compressed to the operating pressures required for conversion to a hydrocarbon product stream by contact with an FTS catalyst. The resulting hydrocarbon stream is then sent to a separator wherein a liquid hydrocarbon stream is separated which is then fractionated to various hydrocarbon grades, including a gasoline grade hydrocarbon stream.
Since that time, the FTS process has been widely studied and various modifications have been devised to improve its economics for gasoline production. Nevertheless, it is still not feasible to synthetically produce gasoline by an FTS procedure which is cost competitive to petroleum refined gasoline and, absent factors which override economic concerns, it is not commercially practiced. To date, liquid fuel production by FTS is only practiced at those locations to which a reliable supply of refinery grade petroleum crude is unavailable. Today the Republic of South Africa, for instance, produces liquid hydrocarbons from coal/methane by an FTS procedure.
In the early 1970's as the prices of petroleum dramatically rose, researchers, particularly those at Mobil Oil Corporation, developed a class of molecular sieve catalysts which convert methanol and other methoxy containing compounds to olefins (MTO=methanol to olefins) and/or to gasoline grade hydrocarbons (MTG =methanol to gasoline). Methods for production of a methoxy containing starting material, such as methanol, from carbon source materials ranging from wood to coal to methane have long been known.
The destructive distillation of wood is the first process by which methanol was produced. In the mid 1920's, a synthetic method for producing methanol from hydrogen and carbon oxides was developed. This synthetic method was first practiced at high pressures (250-350 atmospheres). By the late 1960's, improvement in the catalyst used for methanol production permitted the more economical production of methanol from hydrogen and carbon oxides at a lower pressure of 50-100 atmospheres (.about.730-1470 psig).
The conventional wisdom prevailing relating to methanol production directs one to convert the carbon source material, whether naphtha or methane, to the required hydrogen and carbon oxides containing synthesis gas by steam reforming. Wherein a methanol plant is designed for operation on a petroleum residue feed stream or coal, to enable greater flexibility to handle feed streams of variable composition, conventional wisdom prevailing directs one to prepare the synthesis gas by partial oxidation of the carbonaceous material feed with essentially pure oxygen.
Though methods for production of methanol from a synthesis gas are known, the relatively low equilibrium conversion of hydrogen and carbon monoxide to methanol requires a large recycle of the unreacted hydrogen and carbon monoxide in order to obtain a high efficiency of conversion of carbon input into methanol product. This significantly increases the size of equipment, and therefore the capital cost of a coupled methane to methanol to gasoline operation. Fortunately, the Mobil methanol to gasoline conversion catalyst was also found to be operative to convert dimethyl ether (DME) to gasoline. Accordingly, one means for improving the economics for a methane to methanol to gasoline operation is to convert methane to a mixture of methanol and dimethyl ether to enhance the efficiency of the conversion of carbon input into methoxy carbon compounds which the Mobil catalyst can convert to gasoline. See for example U.S. Pat. No. 3,894,102.
With the development of Mobil's MTG process and the continuing escalation of crude oil prices, during the 1970-early 80's, it appeared that production of gasoline from methane at a cost competitive to refined gasoline could be accomplished by coupling a conventional methanol production plant front end to a Mobil MTG process as the finish end.
In the early 1980's New Zealand, which then depended for its gasoline supply totally on imported crude oil products, undertook at a cost of about 1.2 billion dollars to construct a plant for production of gasoline from methane. The overall plant design comprised two main units, one for the production of methanol from methane, and the second using the Mobil MTG technology for converting methanol to gasoline. In effect, the New Zealand synthetic gasoline plant is two separate plants built side-by-side on common grounds. The first plant is a standard methanol production plant to produce the methanol which is the raw feed required for the Mobil MTG plant. In the Mobil MTG plant, a portion of the methanol feed is converted to DME and this product stream is then converted to gasoline over a molecular sieve catalyst.
In the design of the New Zealand synthetic gasoline plant, the synthesis gas necessary for the production of methanol is prepared by steam reforming natural gas at a pressure of less than 20 atmospheres (.about.290 psig). The equipment required for steam reforming is both high in capital and operating costs. The synthesis gas is then compressed to a pressure of 100 atmospheres (.about.1470 psi), a procedure which requires high capital and operating cost compressing equipment, but which is necessary to the proper operation of a methanol conversion unit on such synthesis gas. In view of the then high and steady rise of crude oil prices experienced in the late '70's and early '80's, the high capital cost associated with steam reforming and compression of the synthesis gas to produce the methanol needed was not seen as a prohibitive economic disadvantage to installation of the synthetic gasoline plant.
Installation of the New Zealand plant was complete and operations commenced in 1985. At that time, crude oil prices had fallen significantly from the level they had earlier attained and synthetic gasoline produced by the New Zealand plant was, and still is, economically uncompetitive with the price of refined gasoline; in major reason because of the cost, both capital and operating, associated with producing methanol from methane.
The Mobil molecular sieve catalyst process for converting methanol and/or dimethyl ether (DME) to gasoline is attractive provided that methanol and/or dimethyl ether can be made available at a practical cost.
In an attempt to improve the economies of synthetic gasoline production using the Mobil MTG process Haldor Topsoe developed a process now commonly known as the Tigas process. The Tigas process integrates methanol synthesis and gasoline synthesis into a single process loop which eliminates the separation of methanol as a discrete intermediate product. To accomplish this integration, Tigas combines both strains of conventional wisdom prevailing in standard methanol production operations in order to eliminate the need to compress synthesis gas from a steam reformer to the pressure required for operation of a methanol plant. Accordingly, in the Tigas process, methane is first steam reformed in part at a pressure of about 30 to 50 atmospheres (440-730 psi) to a high CO.sub.2 content precursor synthesis gas and the unreacted methane content of this precursor synthesis gas is then secondarily reformed by partial oxidation with essentially pure oxygen to produce a still CO.sub.2 rich final synthesis gas having a pressure of about 28 to 48 atmospheres (410-700 psi). This final moderate pressure synthesis gas is then sent to a reactor containing a catalyst which is active for producing both methanol and dimethyl ether from the synthesis gas. Although this reactor operates at a somewhat lower pressure than does a methanol only reactor, because of its coproduction of dimethyl ether a high conversion of methane based carbon to combined methanol and dimethyl ether is still obtained. Total conversion of natural gas input carbon to a methoxy compound containing feed stream composition upon which the Mobil MTG process can operate is high. The methanol and dimethyl ether containing product gas stream is then reacted over a Mobil catalyst to convert the methoxy compounds thereof to liquid hydrocarbon compounds which are separated from the product gas stream and a portion of the residual overhead gasses containing unreacted hydrogen, carbon dioxide, methanol, ethane and olefins are recycled back to the inlet of the methanol/dimethyl ether reactor.
Although the Tigas design somewhat improves the economics for synthetic gasoline production from methane, it still requires a high capital cost steam reforming unit to which Tigas adds a requirement for a high capital cost oxygen plant to permit secondary reforming. The high capital cost required for a synthesis gas compressor is eliminated by Tigas in favor of a high capital cost oxygen plant to obtain in the tradeoff, a net reduction of capital and operating cost, after the obtainment of the synthesis gas, in the form of units of smaller duty size down stream. Though an improvement, the Tigas process is not economically feasible for synthetic gasoline production from methane in light of its high attendant capital cost.
Some variations to the basic Tigas process have been reported to further reduce the need for high capital cost items. One such variation is reported in U.S. Pat. No. 4,481,305. In this variation, an improvement in the economies of recycle is reported to be obtained compared to the standard recycle procedure described by U.S. Pat. No. 3,894,102 to be used with the Mobil MTG process. The improvement requires that adjustments be made to the composition of the synthesis gas feed to a methanol/dimethyl ether production reactor such that the synthesis gas feed will contain carbon monoxide and hydrogen in a CO/H.sub.2 ratio of above 1 and contain carbon monoxide and carbon dioxide in a CO/CO.sub.2 ratio of from 5 to 20. A synthesis gas of such composition may be obtainable from coal or a similar carbonaceous starting material. It is, however, not economically feasible to prepare a synthesis gas of such composition from methane.
Even though synthetic gasoline production processes such as FTS, standard Mobil and/or Tigas have undergone steady improvements intended to render them more economical to the production of synthetic gasoline from methane, they are today still unable to produce gasoline at a cost competitive to that refined from petroleum crude. This is so, even where a source of low cost methane is conveniently located to or transportable to the synthetic gasoline production plant site.
Natural gas resources are located in many areas which are remote from means for transporting such natural gas conveniently and/or economically to markets. In many such remote locations, the natural gas is produced in association with crude oil production and the so-produced natural gas must be disposed of, by flaring or reinjection, in order to produce the crude. Flaring has become an unacceptable method of disposal since it is an economic waste of a diminishing hydrocarbon resource and is also a source of air pollution. Reinjection, which adds to the cost of crude oil production, is often unacceptable both in view of its cost and the adverse effects it may impose upon crude oil production from the field itself.
The inability to dispose of natural gas produced in association with crude at a remote location in a manner which is economically, governmentally and environmentally acceptable has brought crude oil production at some locations to a halt. Application of a currently existing process for conversion of such remotely located natural gas to methanol and/or for synthetic gasoline production from such remotely located natural gas is not economically feasible in view of the great capital cost associated with the equipment necessary to practice such processes.
A process for converting natural gas to methanol, dimethyl ether or gasoline which is feasible for practice from the standpoint of the capital and operating cost associated thereto is a highly desired goal.